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In the presence of multiple bands, well-known electronic instabilities may acquire new complexity. While multiband superconductivity is the subject of extensive studies, the possibility of multiband charge density waves (CDWs) has been largely ignored so far. Here, combining energy dependent scanning tunnelling microscopy (STM) topography with a simple model of the charge modulations and a self-consistent calculation of the CDW gap, we find evidence for a multiband CDW in 2H-NbSe2. This CDW not only involves the opening of a gap on the inner band around the K-point, but also on the outer band. This leads to spatially out-of-phase charge modulations from electrons on these two bands, which we detect through a characteristic energy dependence of the CDW contrast in STM images., While multiband superconductivity is the subject of extensive studies, the possibility of multiband charge density waves (CDW) remains elusive. Here, the authors report evidence of gap opening on both inner and outer bands by a CDW state in 2H-NbSe2, suggesting a possible multiband CDW.

We report a detailed study of the microscopic effects of Cu intercalation on the charge density wave (CDW) in 1T-CuxTiSe2. Scanning tunneling microscopy and spectroscopy reveal a unique, Cu-driven spatial texturing of the charge-ordered phase, with the appearance of energy-dependent CDW patches and sharp π-phase shift domain walls (πDWs). The energy and doping dependencies of the patchwork are directly linked to the inhomogeneous potential landscape due to the Cu intercalants. They imply a CDW gap with unusual features, including a large amplitude, the opening below the Fermi level, and a shift to higher binding energy with electron doping. Unlike the patchwork, the πDWs occur independently of the intercalated Cu distribution. They remain atomically sharp throughout the investigated phase diagram and occur in both superconducting and nonsuperconducting specimens. These results provide unique atomic-scale insight into the CDW ground state, questioning the existence of incommensurate CDW domain walls and contributing to understanding its formation mechanism and interplay with superconductivity.

The copper oxides present the highest superconducting temperature and properties at odds with other compounds, suggestive of a fundamentally different superconductivity. In particular, the Abrikosov vortices fail to exhibit localized states expected and observed in all clean superconductors. We have explored the possibility that the elusive vortex-core signatures are actually present but weak. Combining local tunneling measurements with large-scale theoretical modeling, we positively identify the vortex states in YBa2Cu3O7-δ. We explain their spectrum and the observed variations thereof from one vortex to the next by considering the effects of nearby vortices and disorder in the vortex lattice. We argue that the superconductivity of copper oxides is conventional, but the spectroscopic signature does not look so because the superconducting carriers are a minority.

We study the impact of Cu intercalation on the charge density wave (CDW) in 1T-CuxTiSe2 by scanning tunneling microscopy and spectroscopy. Cu atoms, identified through density functional theory modeling, are found to intercalate randomly on the octahedral site in the van der Waals gap and to dope delocalized electrons near the Fermi level. While the CDW modulation period does not depend on Cu content, we observe the formation of charge stripe domains at low Cu content (x < 0.02) and a breaking up of the commensurate order into 2 × 2 domains at higher Cu content. The latter shrink with increasing Cu concentration and tend to be phase shifted. These findings invalidate a proposed excitonic pairing as the primary CDW formation mechanism in this material.

To celebrate the International Year of Crystallography among the general public, a consortium of chemists, physicists and crystallographers of the University of Geneva organised in Spring 2014 an incentive crystal growth contest for Geneva scholars aged 4 to 19. Starting from a kit containing a salt and user instructions, classes had to prepare a crystal that met specific criteria according to their category of age. The composition of the salt – potassium dihydrogen phosphate (KDP) – was only disclosed to the participants during the final Awards Ceremony. This contest positively exceeded our expectations with almost 100 participating classes (ca. 1800 participants) and 54 specimens received over all categories.

The treatment of multiple myeloma has undergone significant changes in the recent past. The arrival of novel agents, especially thalidomide, bortezomib and lenalidomide, has expanded treatment options and patient outcomes are improving significantly. This article summarises the discussions of an expert meeting which was held to debate current treatment practices for multiple myeloma in Switzerland concerning the role of the novel agents and to provide recommendations for their use in different treatment stages based on currently available clinical data. Novel agent combinations for the treatment of newly diagnosed, as well as relapsed multiple myeloma are examined. In addition, the role of novel agents in patients with cytogenetic abnormalities and renal impairment, as well as the management of the most frequent side effects of the novel agents are discussed. The aim of this article is to assist in treatment decisions in daily clinical practice to achieve the best possible outcome for patients with multiple myeloma.

Using scanning tunneling spectroscopy, we investigated the temperature dependence of the quasiparticle density of states of overdoped Bi2Sr2CuO6+δ between 275 mK and 82 K. Below Tc=10K, the spectra show a gap with well-defined coherence peaks at ±Δp≃12meV, which disappear at Tc. Above Tc, the spectra display a clear pseudogap of the same magnitude, gradually filling up and vanishing at T∗≃68K. The comparison with Bi2Sr2CaCu2O8+δ demonstrates that the pseudogap and the superconducting gap scale with each other, providing strong evidence that they have a common origin.

We present a scanning tunneling spectroscopy study on quasiparticle states in vortex cores in Bi2Sr2CaCu2O8+δ. The energy of the observed vortex core states shows an approximately linear scaling with the superconducting gap in the region just outside the core. This clearly distinguishes them from conventional localized core states and is a signature of the mechanism responsible for their discrete appearance in high-temperature superconductors. The energy scaling of the vortex core states also suggests a common nature of vortex cores in Bi2Sr2CaCu2O8+δ and YBa2Cu3O7−δ. Finally, these states do not show any dependence on the applied magnetic field between 1 and 6 T.

We present a detailed study on the behavior of vortex cores in Bi2Sr2CaCu2O8+δ using scanning tunneling spectroscopy. The very irregular distribution and shape of the vortex cores imply a strong pinning of the vortices by defects and inhomogeneities. The observed vortex cores seem to consist of two or more randomly distributed smaller elements. Even more striking is the observation of vortex motion where the vortex cores are divided between two positions before totally moving from one position to the other. Both effects can be explained by quantum tunneling of vortices between different pinning centers.

Vortex cores in under- and overdoped Bi2Sr2CaCu2O81d are studied by local probe tunneling spectroscopy. At the center of the cores, we find a gaplike structure at the Fermi level which scales with the superconducting gap, but no quasiparticle bound states. This low temperature pseudogap is intimately related to the superconducting gap and shows striking similarities with the normal state pseudogap measured above Tc. A possible interpretation is that both pseudogap structures reflect the same “normal” state containing phase incoherent excited pair states.

We present tunneling spectroscopy of Bi2Sr2CaCU2O8+delta Single crystals as a function of oxygen doping and temperature. The doping dependence amounts essentially to an energy scaling of the tunneling spectra, with a quasiparticle gap that is reduced with increasing oxygen concentration. This superconducting gap is temperature independent up to the superconducting transition where the superconducting spectra merge continuously into another gaplike feature at the Fermi level. This pseudogap is found to be present both in underdoped and overdoped samples, and it scales with the superconducting gap.

We report the observation of the flux line lattice in YBa2Cu3O7−δ by scanning tunneling microscopy. The measurements were carried out at 4.2 K and in a magnetic field of 6 T applied along the c axis. The vortices appear arranged in an oblique lattice in which the primitive vectors are nearly equal and form an angle of approximately 77°. We also report local tunneling spectroscopy into a vortex core which reveals two peaks separated by about 11 meV. The zero-field spectra are reproducible over large areas of the sample and show a multiple peak structure.

This paper reports very detailed low-temperature vacuum tunneling spectroscopy investigations of Bi2Sr2CaCu2O8+δ (BSCCO) single crystals. For clean vacuum junctions formed between a cleaved BSCCO single crystal and the normal-metal tip of a scanning tunneling microscope, we obtain stable c-axis vacuum tunneling conditions that allow very reproducible low-temperature electron tunneling spectroscopy. A vacuum junction is identified by tunneling spectra which neither depend on tip/sample spacing nor change as a function of time and position on the sample in the Meissner state. In contrast to the frequently reported linear or parabolic increase with increasing bias voltage, the background conductance of such spectra is largely constant with a slight decrease up to ±300 meV bias. The normal-state conductance inferred from this background has a local maximum at negative sample bias, indicating a pileup below the Fermi level in the (ab)-plane normal-state density of states of BSCCO. Vacuum junctions at 4.8 K show a well developed superconducting gap with large peaks at the gap edges and a finite density of quasiparticle excitations filling the gap. These characteristics are not consistent with an isotropic BCS-like gap parameter. Outside the superconducting gap, the differential conductance curves are very asymmetric, with a striking dip which appears at negative sample bias only. This dip contributes a substantial amount of states to satisfy the conservation of states we find between normal and superconducting BSCCO.

A study of enhanced field emission on broad area niobium cathodes is presented. Emitting sites have been investigated by field emission scans, scanning electron microscopy and scanning tunneling microscopy. All except one of 14 studied emitters are of micron or submicron size. A trend is observed for the emitting structures to be smaller than predicted from the protrusion model as is also demonstrated with the aid of a specific example of a field emitting site.

Enhanced field emission on high-purity aluminum has been investigated. The current/voltage characteristic (Fowler-Nordheim plot), the chemical composition as determined by Auger electron spectroscopy and X-ray microprobe analysis, and the topography of field-emitting sites were studied locally. In addition, results on the evolution of these sites under heat treatment, Ar/sup +/ sputtering, and exposure to atmosphere are presented. The field-emitting sites on aluminum are all related to particles approximately 30 mu m in size standing on the surface. The average emission over 1-cm/sup 2/ areas is little affected by the abovementioned surface treatments.

We study high temperature superconductivity of heavily overdoped (p~0.22) Bi2Sr2CaCu2O8+d with the objective to find signatures of the more conventional superconductivity anticipated in comparison to its more widely studied underdoped counterpart. We find some similarities as well as distinctly different properties, in particular a short-range static periodic modulation caused by quasiparticle interference (QPI) due to scattering off smooth disorder, without invoking density waves. Our study further lifts a long-standing puzzle about the electronic vortex core structure in this compound. We find convincing evidence that QPI is also responsible for low-energy periodic modulations reported inside vortex cores at high magnetic fields (B=3T). Reducing the applied magnetic field by over an order of magnitude (B=0.16T) nearly completely suppresses QPI and vortex-vortex interactions, revealing spectral signatures in these more isolated vortex cores that are in striking agreement with BCS expectations.

In this work, we performed a detailed scanning tunnelling microscopy and spectroscopy study of the charge density wave state of 1T-CuxTiSe2 in a wide range of Cu concentrations (0 < x < 0.07), an apparently perfect playground for investigating the interplay between correlated ground states. Thanks to density functional theory modelling, harmonic wave simulations and defect analysis, we were able to resolve for the first time the charge density wave contributions from the full Se-Ti-Se trilayer with subatomic precision, and to show the excellent agreement with the proposed periodic lattice distortion models. By changing the energy integration windows in our measurements, we reported perfect contrast inversion of the charge density wave signal, as expected by a simple mean-field BCS-like theory and by density functional theory modelling. Conversely, we found a mismatch in the energies at which this contrast inversion takes place between the theoretical value (Eci=EF) and the experimental one (Eci ~ -200 mV), which together with spectroscopic data suggest that in this system the charge density wave formation does not yield a gap opening at the Fermi energy, but at slightly higher binding energies. Moreover, the gap position is extremely sensitive to external perturbations such as surface adsorbates and chemical doping. In fact, by adding Cu atoms to the system, we reported an unprecedented energy-dependent patchwork of charge density wave regions, closely related to the local Cu content, to the consequent changes in the potential lanscape and, accordingly, to the charge density wave gap position. The reported resilience of the charge density wave gap further allows the quantum condensate to endure strong external perturbations, such as chemical doping, the loss of long-range order induced by exciton melting in metallic samples and the formation of superconductivity.

The charge density wave (CDW) phase in bulk and exfoliated crystals of transition metal dichalcogenides was studied by scanning tunneling microscopy (STM). In exfoliated flakes of VSe2 we found a striking non-monotonic evolution of the CDW transition temperature as function of thickness derived from the analysis of the real space charge modulation amplitude imaged by STM. This finding lifts the contradiction of previous measurements. In bulk crystals of VSe2, CuxTiSe2 and NbSe2 we examined the CDW phase by our original method to extract the local amplitude, wavelength and phase of the CDW. This high resolution spatial mapping of the full complex order parameter provided unprecedented insight into fundamental aspects of CDWs. It experimentally proved that the CDW in these materials consist of three individual charge modulations. Furthermore, it explained the different contrasts often observed in STM images and revealed a rich variety of features like phase domains and topological defects.

Manganese self-assembles into atomic chains on the Si(001) reconstructed surface. Our combined scanning tunneling microscopy, atomic force microscopy (AFM) and density functional theory (DFT) study revealed a simple necklace-like structure of the Mn chains. The DFT calculated adsorption energies support a single Mn atom adsorption H' site, however no gain in energy promoting the formation of chains was found. We use atom manipulation techniques to deconstruct the chains and confirm the H' site model. Moreover, we show that the manipulated atoms dissipate energies, in the damping channel of AFM, that can be compared to the diffusion energies from DFT calculations.

We present a scanning tunnelling microscopy study of the metal insulator (MI) phase transition in NdNiO3 thin films grown on NdGaO3. Although topographic resolution is limited to grain structures and step edges, we did achieve high resolution and reproducible spectroscopic mapping of the thin film surface. The spectra we measure are either metallic or insulating, and mapping the local gap as a function of position shows the emergence of well-defined insulating domains in a conducting background upon cooling the film through the MI transition. When warming the film through the MI transition, the opposite is observed, with metallic domains appearing in the insulating background. There is no gradual shift in the measured insulating gap amplitude at the phase transition. The measured proportion of metallic surface area as a function of temperature follows the resistive hysteresis loop and allows a direct comparison with a percolation model for the MI phase transition.

The interplay between charge order and superconductivity has been a long standing puzzle in a wide range of superconducting materials. The transition metal dichalcogenide 1T-TiSe2 is an interesting material that can give an important contribution to this debate. Indeed, 1T-TiSe2 undergoes a charge density wave transition and does also become superconducting upon Cu intercalation with a maximum transition temperature of 4.1 K. In this work we present a low temperature scanning tunneling microscopy/spectroscopy study on CuxTiSe2 (0≤x≤0.07). Our main results include the identification of single atom defects as well as Ti and Cu intercalation sites. This provides unique atomic scale information on the local crystal structure (1T vs 2H polytype) and precise registry of the CDW with the atomic lattice. We observe a striking instability of the CDW towards stripe formation. We measure the superconducting gap near optimal Cu intercalation and find no obvious correlation with the local CDW phase.

The mechanism underlying high temperature superconductivity continues to challenge our understanding. Scanning tunneling spectroscopy (STM) has historically been one of the prime techniques to tackle superconductivity in conventional BCS materials, providing key insight into the pairing mechanism. Here we present a new set of tunneling spectra obtained on YBCO single crystals. Our new results are twofold: Firstly, we have observed low energy electronic states that previously have been associated with the vortex core in HTS, to be present everywhere on the sample surface, both inside and outside vortices. This fact suggests that these features have a more general origin and are not specific signatures of the vortex core. Secondly, we have found these low energy states to be modulated in space, with a nematic order.

Dans ce travail, nous nous sommes intéressé à l'auto assemblage de nanolignes de bismuth sur la surface du silicium et des structures dérivées de ces nanolignes. Nous avons découvert que l'hydrogénation permet d'enlever complètement le bismuth de la surface tout en conservant la reconstruction unidimensionnelle des atomes de silicium supportant ces nanolignes. Cette structure, appelée bande de Haiku (Haiku stripe), est composée uniquement de silicium recouverte d'une monocouche d'hydrogène. Elle est incorporée aux terrasses de la surface du Si(001) et est stable à l'air. Ces nanolignes ont une largeur fixe de 4 dimères de Si pour une longueur pouvant dépasser 4 μm. Nous avons caractérisé structurellement et électroniquement les bandes de Haiku par des mesures de microscopie à effet tunnel validées ensuite par des simulations théoriques. La présence d'un état unidimensionnel confiné au centre de la structure des bandes de Haiku a été démontrée. La réunion de toutes ces caractéristiques structurelles et électroniques uniques fait des bandes de Haiku un système des plus attractifs pour la réalisation d'un système modèle pour l'étude de la physique à basse dimension.